7. GALAXY IDENTIFICATIONS

Because of the presence of bright, B ~ 18.5, background QSOs,
surveys for galaxies associated with high-z damped
Ly systems are more
challenging than surveys for randomly selected galaxies. To illustrate
this point consider a sightline passing through an
L* galaxy
6 at a reasonable impact
parameter of 10 kpc. At z = 3 the galaxy will have an AB
magnitude of 24.7 and an impact parameter of 1.3 arcsec. Detection of
the galaxy against the QSO PSF with ground-based telescopes would be
exceedingly difficult even under the best seeing conditions and has even
proven difficult with broadband-space imaging. Using the NICMOS IR
camera on the HST,
Colbert & Malkan (2002)
surveyed 22 damped Ly
systems and detected only one candidate counterpart down to
HAB = 23.5, implying that most damped
Ly systems are not drawn
from the luminous end of the Lyman Break Galaxy luminosity function.
Warren et al. (2001)
probed even deeper, to HAB = 25, and found 41
candidate counterparts near 18 high-redshift damped
Ly systems. Broadband
imaging to the necessary depth is thus limited by source confusion
within reasonable impact parameters.

The most widely used techniques are searches for
Ly emission lines at the
absorption redshift. The advantage of this method is that the wavelength
of
Ly emission is located
at the bottom of the damped
Ly absorption trough,
which blocks the bright light of the background QSO. As a result,
background night sky emission is the only source of external
noise. Using slit spectroscopy,
Foltz, Chaffee &
Weymann (1986)
failed to detect Ly
emission with a 3 upper
limit of F ~ 10-16 ergs cm-2
s-1 for an unresolved object. While
Hunstead, Fletcher &
Pettini (1990)
claimed detection of Ly
emission from a compact source coinciding with DLA0836 + 11 at z
= 2.466, this feature was not confirmed in spectra acquired by
Wolfe et al. (1992)
and Lowenthal et
al. (1995).
Nor was Ly emission
detected in imaging surveys using narrow-band interference filters or
Fabry-Perot interferometers. In this case the QSO light is blocked
because the bandwidth of the filter is centered on the damped
Ly line but has a
narrower FWHM. This technique is ideal for impact parameters large
compared to the seeing radius, since an emitter located outside a slit
could still be detected in the narrow-band image.
Smith et al. (1989),
Deharveng, Bowyer &
Buat (1990)
and Wolfe et al. (1992)
carried out narrow-band surveys for
Ly emission.
Deharveng, Bowyer &
Buat (1990),
Wolfe et al. (1992),
and Lowenthal et al. (1995)
carried out Fabry-Perot surveys. No detections to limiting fluxes of
F 5 ×
10-17 ergs cm-2 s-1 for unresolved
objects were reported. The extended
Ly emitter associated
with DLA0836 + 11 at z = 2.466 claimed by
Wolfe et al. (1992)
is more likely to be a galaxy associated with a lower redshift Mg II
absorption system
(Lowenthal et al. 1995).

The failure to detect
Ly emission could result
from the destruction of resonantly trapped photons by even a small
amount of dust
(Charlot & Fall 1991).
For this reason several groups attempted to detect damped
Ly systems in
H emission since
H photons are not
resonantly trapped.
Bunker et al. (1999)
used IR detectors on a 4 m class ground-based telescope to search for
H in five damped
Ly systems, but none was
detected (see also
Mannucci et al. 1998).
Of course, the failure to detect
H emission might also be
due to small impact parameters, since an emitting region located within
the PSF of the QSO would not be detected from the ground. However,
Kulkarni et al.
(2000,
2001)
used NICMOS to search for
H emission from two
damped Ly systems and
none was found (see Table 1).

With the recent detections of
Ly emission from at
least 2 out of 18 damped
Ly systems surveyed
using 8- to 10-m class telescopes
(Møller et al. 2002;
Møller, Fynbo &
Fall 2004),
it is evident that the previous null detections of
Ly were largely due to
the lower sensitivity of 4-m class telescopes. The results, summarized
in Table 1, indicate that two of the three known
Ly emitters, DLA0458 -
02 and DLA0953 + 47A, would not have been detected in the earlier
surveys. While the sample is too small to draw general conclusions, the
results are interesting for the following reasons. First, damped
Ly systems resemble
randomly selected
Ly emitters by the
similarity in
Ly luminosity and in the
compact size of the emission regions. Second, the small impact
parameters for DLA0458 - 02 and DLA0953 + 47A suggest that the H I
absorbing layers are smaller than ~ 5 kpc. However, since the H I
content of DLA0458 - 02 is known to extend over linear scales exceeding
17 kpc
(Briggs et al. 1989),
the sightline to the QSO must pass close to a compact star forming
region, which is embedded in a much larger layer of H I. Third, no
continuum emission has been detected from the same two damped
Ly systems. The B
> 27 limit on DLA0953 + 47A places this damped
Ly system near the faint
end of the known luminosity function of Lyman Break Galaxies.

By contrast, DLA2206 - 19A is a luminous Lyman Break Galaxy. This
emitter was first located in IR images obtained with NICMOS
(Warren et al. 2001).
The STIS image
(Møller et al. 2002)
shows rest-frame FUV stellar emission extending 1 arcsec between a
bright knot and the QSO sightline.
Ly emission at the
redshift of the damped
Ly system was detected
from the knot with spectra obtained with the VLT
(Møller et al. 2002).
The magnitude integrated over the object is V = 23
(P. Møller 2004, priv. comm.), which places this damped
Ly system at the bright
end of the Lyman Break luminosity function.

As a result, there is little overlap between the luminosity functions of
damped Ly systems and
the R < 25.5 spectroscopic sample of Lyman Break Galaxies (see
Møller et al. 2002,
Schaye 2001).
Efforts to detect the clustering of damped
Ly systems with
neighboring Lyman Break Galaxies have so far only yielded upper limits
(Adelberger et al. 2003,
Gawiser et al. 2001),
providing further evidence that the damped
Ly systems are nearly
disjoint from the R < 25.5 spectroscopic sample of Lyman Break
Galaxies, with DLA2206 - 19A a clear exception. Based on the UV continua
implied by the
Ly luminosities of the
other two damped
Ly systems detected in
emission, there is growing evidence that at least some damped
Ly systems overlap with
the dimmer "photometric" sample of Lyman Break Galaxies at R <
27 seen in the Hubble deep fields.

While the nature of the galaxies associated with damped
Ly systems at z
> 1.6 is still unclear, at lower redshifts there should be a close
resemblance to objects drawn from the population of normal galaxies if
damped
Ly systems trace the
star-formation history of normal galaxies. However, the low
metallicities inferred for most low-z damped
Ly systems
(Kulkarni et al. 2004,
Pettini & Steidel
1999)
has cast doubt on this idea and has led to the suggestion that
low-z damped
Ly systems are
metal-poor objects such as dwarf galaxies
(Calura, Matteucci &
Vladilo 2003)
or low surface-brightness galaxies
(Jimenez, Bowen &
Matteuci 1999).
As a result, identification of galaxies associated with damped
Ly systems at z
< 1 is of vital importance.

There are 23 damped
Ly systems known at
z < 1.6 (as of October 2004). Thirteen of these have been
identified using spectroscopic or photometric redshift techniques
(Chen, Kennicutt & Rauch 2004;
Chen & Lanzetta 2003)
and possible host galaxies for additional eight systems have been found
in imaging surveys
(Le Brun et al. 1997,
Rao et al. 2003).
Chen & Lanzetta (2003)
analyzed an unbiased subset of nine galaxies with redshifts and
concluded that the resultant luminosity distribution was dominated by
luminous galaxies with L / L* >
0.1, i.e., luminous galaxies dominate the neutral gas cross-section at
z < 1. More specifically, they used a maximum-likelihood
technique to determine the dependence of H I cross-section on luminosity
and showed that a cross-section weighted Schechter function with typical
parameters for normal galaxies provided a good fit to the data.

On the other hand,
Rao et al. (2003)
analyzed a heterogeneous sample of 14 damped
Ly system host galaxy
candidates, including objects without confirmed redshifts and concluded
that the neutral gas cross-section at z < 1 was dominated by
dwarf galaxies. Since
Rao et al. (2003)
did not quantify their result, this conclusion is difficult to
evaluate. However, comparison with the sample of
Chen & Lanzetta (2003)
shows a larger fraction of galaxies with lower values of
L/L* in the
Rao et al. (2003)
sample. In some cases these are galaxies without confirmed redshifts at
small angular separations from the damped
Ly sightline. In other
instances, where two or more galaxies are found to be associated with
the damped
Ly system,
Rao et al. (2003)
chose the lowest
luminosity galaxy for the analysis because it had the smallest impact
parameter. However,
Chen & Lanzetta (2003)
argue that in these cases, an entire group of galaxies is responsible
for the absorption profile, so the object with lowest impact parameter
may not be the appropriate choice. Given the uncertainties arising from
the small number of objects, we conclude that the current data are
consistent with the galaxies associated with low-redshift damped
Ly systems being drawn
from the population of normal galaxies.